I believe I have solved this issue, but it was a frustrating experience for me. I tried several USB drives, different OS installs (which worked just fine), and removal of the hardware in the expansion slots to try and figure out the issue. Turns out it was very simple, and I just had to be there during the very long boot process.
Issue: Qubes installer will act as normal, except for the test media boot option, and let you proceed beyond the language and keyboard selection screen. When you get to the install screen where you select the additional TemplateVM you want, the timezone, and the installation destination, the installer will say "Probing install media" and will stay stuck on this screen. The mouse is still moveable, and one can click the quit button with action taking place but no matter how long the machine is left on this screen, it will not allow the installation process to continue. There are no warnings or messages indicating that there is any issue. (my) cause: One HDD had a S.M.A.R.T. event(drive is starting to fail), unbeknownst to me, and the removal of this drive fixed the issue and allowed the installation process to complete. -- You received this message because you are subscribed to the Google Groups "qubes-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to qubes-users+unsubscr...@googlegroups.com. To post to this group, send email to qubes-users@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/qubes-users/00243006-7e29-4071-b070-79b72efa586f%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.